$\begingroup$@Seth CIR is the Carrier to Interference Ratio. I have obtained it by taking out the Interfernce in dBm to the received power in dBm.$\endgroup$
– pabloApr 18 '18 at 19:44

$\begingroup$Hint: I have no idea where you got your formula from but try rewriting it as $$\frac{1}{SNIR} = \frac{1}{SNR}+\frac{1}{CIR}$$ and see if that suggests the way to proceed further.$\endgroup$
– Dilip SarwateApr 18 '18 at 20:04